Such a transponder with the corresponding reader interrogator was described in, e.g., the prospectus Elektronische Identifikation System Trovan Trovan Electronic Identification System! of the firm AEG Aktiengesellschaft, Ulm, of June 1992. The transponder described there has a fixed-code memory as the useful memory. If the transponder is located in the field range of the reader, it is prompted to transmit the contents of its useful memory, as a result of which it is identifiable for the reader.
In the case of applications in which a plurality of transponders may be simultaneously in the field range of the reader, this simple arrangement does not operate with sufficient reliability, because the signals of a plurality of transponders transmitting simultaneously cannot be, in general, separated and evaluated, at least not without substantial extra cost.